Surface modification of boron nitride by reduced graphene oxide for preparation of dielectric material with enhanced dielectric constant and well-suppressed dielectric loss

Composites Science and Technology 134 (2016) 191e200

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Surface modification of boron nitride by reduced graphene oxide for preparation of dielectric material with enhanced dielectric constant and well-suppressed dielectric loss Kai Wu a, Chuxin Lei a, Weixing Yang a, Songgang Chai b, Feng Chen a, **, Qiang Fu a, * a b

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China Guangdong Shengyi Technology Limited Corporation, Dongguan, 523039, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 July 2016 Received in revised form 12 August 2016 Accepted 18 August 2016 Available online 20 August 2016

Adding conductive filler is an effective way to enhance the dielectric constant while usually also increases the dielectric loss of polymer. In this study, we demonstrated that polymer composites with much improved dielectric constant while maintaining ultra-low dielectric loss could be achieved via using hybrid filler and controlling the dispersion of conductive filler in polymer matrix. To do this, the graphene oxide was designed to be immobilized on the surface of large-sized insulating hexagonal boron nitride (h-BN) via electrostatic self-assembly, and afterwards introducing this hybrid filler into epoxy accompanied with chemical reduction. In this case, since the reduced graphene oxide (rGO) sheets were fixed on the surface of h-BN, rGO sheets were well separated from each other even at high loading. Hence not only significantly enhanced dielectric constant was observed, but also a very low dielectric loss comparable to that of neat epoxy was achieved. This low dielectric loss was believed to be ascribed to both embedded insulating network of h-BN to inhibit the mobility of charge carrier and well-separated rGO sheets via immobilization. In addition to obviously improved dielectric properties, the nanocomposites also exhibited good thermal conductivity. We believe that this special structure will provide a new thought for fabricating dielectric materials with much enhanced dielectric constant as well as wellsuppressed dielectric loss. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Polymer-matrix composites Thermal properties Electrical properties

1. Introduction Polymer composites with high dielectric constant and low dielectric loss have attracted tremendous attention for their fascinating applications in electromechanical actuators, high-density electronic packaging technology and capacitors for energy storage [1e4,53,54]. Additional to the excellent dielectric property, good thermal conductivity is also very important, since the heat generated during service will cause shortening of life time and performance degradation [5e8]. However, most polymers exhibit low dielectric constant and insufficient thermal conductivity, hence multifunctional fillers should be introduced to enhance both the properties [9e15]. Graphene, an ultrathin and flexible layer of sp2-hybridized

* Corresponding author. ** Corresponding author. E-mail address: [email protected] (Q. Fu). http://dx.doi.org/10.1016/j.compscitech.2016.08.015 0266-3538/© 2016 Elsevier Ltd. All rights reserved.

carbon atoms arranged in a two-dimensional hexagonal lattice, has shown attractive prospects for preparing polymer based nanocomposites owing to its extraordinary carrier mobility, excellent mechanical properties and unique thermal conductivity [16e19]. It was reported that graphene or reduced graphene oxide (rGO) nanocomposites can obtain high dielectric constant at the extremely low graphene loading [20,21]. However, it should be noted that as increase of graphene content, the dielectric loss which degrades the discharged energy density and the charge-discharge efficiency will upgrade sharply. Hence innovative strategies are imminently needed to reduce the dielectric loss of graphene nanocomposites [13,22,23]. Recent studies have shown that encapsulating conductive fillers with insulating layers, such as polyaniline (PANI) [13] and titanium dioxide@titanium diboride [24], was beneficial to prevent the direct contact of conductive filler, leading to repressed dielectric loss. For example, Li reported insulating PANI decorating rGO in poly(methyl methacrylate) (PMMA) matrix [13]. It was found that insulating PANI could exert isolation effect on the rGO sheets in

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PMMA to hinder the direct electrical contact between rGO, thus resulting in much lower dielectric loss than that of PMMA/rGO. However, compared to that of neat PMMA, the dielectric loss of PMMA/PANI@rGO still increased to 0.12, over 2.4-fold higher than that of neat PMMA. Thus fabricating polymer/graphene or rGO composites with much improved dielectric constant while maintaining ultra-low dielectric loss (comparable to or lower than that of neat polymer matrix) remains great challenge. Recently, embedded hexagonal boron nitride (h-BN) and exfoliated h-BN in polymer matrix were reported to be effective to inhibit the mobility of the charge carrier, thus resulting in dramatic decrease of dielectric loss [8,25]. Meanwhile, due to its extraordinary antioxidation stability, high thermal conductivity, good mechanical properties and high breakdown strength (around 800 MV m-1), h-BN is considered as one of the best choice for preparation of polymer composites with enhanced thermal conductivity and decreased dielectric loss [5,26e29,55]. To further improve the dielectric constant, the simple way is directly mixing h-BN and graphene or rGO in polymer matrix. In this case, only a small amount of graphene could be used to maintain the low dielectric loss. Otherwise, graphene sheets will contact with each other to degrade the dielectric loss. Thus the key is to find a way which could well separate graphene sheets even at high loading. This need a careful design of interaction between h-BN sheets and graphene sheets, and special structure and combination between these two fillers to control the distribution of graphene sheets in polymer matrix. To do this, in this study, we designed a structure through immobilizing rGO sheets on the surface of large-scale planar lamellar h-BN for the purpose of inhibiting agglomeration of rGO as well as preventing rGO sheets from directly contacting with each other to form conductive network in the polymer matrix. This combination needs chemical stable h-BN to be highly surficial oxidized and functionalized, thus guaranteeing enough absorbed content of rGO. So far, some methods including plasma treatment [30], using reagents which can generate radicals [31] and hydrothermal oxidation [32], were considered to be effective to introduce hydroxyl groups (-OH). But the oxygen content was limited and most of these modifications were complex or time-consuming. Hence not only intricate the fabricating procedure will be, but also the loading of fixed rGO will be restricted. Recently, our group found an efficient route to oxidize h-BN or exfoliated h-BN to obtain the highly oxidized product whose oxygen content can achieve as high as 10.26%. This modification was carried out through a simple ultrasonication treatment in nitric acid (HNO3), followed by a rapid neutralization to generate eOH, accompanied with the obvious change in color from white to brown yellow. The related work will be published elsewhere. In this work, we mainly utilized this highly hydroxylated h-BN which was grafted by 3aminopropyltriethoxysilane (APS) in order to introduce eNH2 on the surface. Afterwards, modified h-BN was mixed with GO in the aqueous solution in order to make GO be fixed on the h-BN by mutual electrostatic interactions. The successful modification, selfassembly and immobilization were confirmed by XPS, Zeta potential, Raman spectra, SEM and TEM characterizations as well as elemental mapping. Afterwards, this hybrid filler (h-BN@GO) was introduced into epoxy, subsequently being reduced into epoxy/hBN@rGO by the amine curing agent as the epoxy was cured at the high temperature. Due to the excellent thermal conductivity of hBN and high dielectric constant of rGO, this incorporated filler combined both the properties together. On the one hand, it was found that the dielectric constant for epoxy/h-BN@rGO (up to ~15) is much higher than that of epoxy filled with h-BN as well as epoxy filled with directly mixed h-BN and rGO at the same filler concentration. On the other hand, of particular note is that the

dielectric loss decreased as increase of h-BN@rGO loading, resulting in obviously lower dielectric loss (0.0068) than that of neat epoxy (0.0092) at the frequency of 100 Hz when the hybrid filler content was 30 wt%. The phenomenon of extremely low dielectric loss is interesting and the mechanism for the well-improved dielectric properties is concluded as inhibited mobility of charge carriers due to embedded insulating network of h-BN as well as suppressed direct contact and agglomeration of rGO sheets due to immobilization. In addition to obviously improved dielectric properties, the nanocomposites also exhibited good capability of heat dissipation with thermal enhancement factor of 390% at 30 wt% loading in comparison with neat epoxy. This result was much higher than polymers filled with same volume content of common dielectric fillers such as barium titanate [7]. Our method is simple and it provides the new thoughts for preparation of dielectric materials with not only high dielectric constant, ultra-low dielectric loss, but also good capability of heat dissipation.

2. Experimental section 2.1. Materials Graphite powders were purchased from Qingdao Black Dragon graphite Co., Ltd. Micron sized hexagonal boron nitride (average lateral size: 30 mm) was supplied by Qinhuangdao Eno Material Co., Ltd. 3-aminopropyltriethoxysilane (APS) was purchased from Aladdin Chemicals Co., Ltd. Potassium permanganate (KMnO4), sulfuric acid (H2SO4 98%), hydrogen peroxide (H2O2), Nitric acid (HNO3), 4, 4-diamino diphenyl methane (DDM), ethanol, tetrahydrofuran (THF) and epoxy (E-44) were supplied by Kelong Chemical reagent plant (Chengdu, China).

2.2. Surface modification of h-BN 500 mg h-BN was dispersed in 500 ml HNO3 solution (65% w/w), followed by sonication for 5 h using an output power of 120 W and a rapid neutralization, as was shown in Fig. 1 (1). Then the product was collected by three washing steps by centrifugation and dried at 60
C under vacuum. This procedure was designed to introduce eOH groups on the surface of h-BN through oxidation, named h-BN (HNO3). Secondly, modified h-BN by HNO3 (500 mg) was mixed with 5 wt% of silane coupling agent APS in the 500 ml ethanol solution (ethanol: H2O ¼ 95:5). Then, the solution was kept at 60
C overnight for reaction. After that, the obtained product named h-BNNH2 was washed by deionized water and ethanol for 2 times, and dried under vacuum.

2.3. Electrostatic self-assembly of h-BN-NH2 and GO Before electrostatic self-assembly, GO was fabricated via the exfoliation of natural graphite by Hummers method [33]. Then the exfoliated GO was collected by centrifugation to remove the nonexfoliated graphite. After that, steady GO aqueous solution was achieved by sonication in deionized water for 2 h. From Fig. 1 (2), the electrostatic self-assembly between GO and h-BN-NH2 was achieved through a simple procedure: 500 ml GO aqueous solution (0.15 mg/ml) and 500 ml h-BN-NH2 suspension (5 mg/ml) were mixed together under mild magnetic stirring for 2 h. Then, the h-BN@GO powder was collected by centrifugation and dried under vacuum.

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Fig. 1. Preparation procedures of epoxy nanocomposites displayed in this schematic: (1) Modification of neat h-BN with HNO3, followed by grafting APS; (2) Self-assembly of GO and modified h-BN in the aqueous solution; (3) The method of fabricate epoxy nanocomposites and the scheme of h-BN@rGO dispersion in the epoxy matrix.

For the purpose of fabricating epoxy/h-BN@rGO, according to Fig. 1 (3), several amounts of h-BN@GO were mixed with epoxy (30 g) and dispersed well under sonication and mechanical stirring in THF. Then, the suspension liquid was exposed to rotary evaporation in order to remove THF. Afterwards, 8.1 g DDM (curing agent) was added and dissolved at 75
C for 15 min by intense stirring, subsequently removing the air bubbles under vacuum. Finally, the mixtures above were poured into die and cured according to the heating sequence: 70
C/1 h, 120
C/2 h, 150
C/2 h. At the same time, the GO was gradually reduced into rGO by DDM at the high temperature. For comparison, epoxy composites with same rGO and hBN content were also prepared by directly adding these two filler into epoxy matrix without self-assembly between them, which is named epoxy/h-BN/rGO nanocomposites.

confirm the immobilization of GO on the surface of h-BN. Zeta potential measurements were characterized using a Zetasizer 3000 (Malvern Instruments). Before measurement, GO, h-BN and modified h-BN aqueous suspensions were respectively diluted to 0.05, 2 and 2 mg/ml. The transient place source (TPS) method was applied to measure the thermal conductivity of the nanocomposites by a Hot Disk thermal analyzer (Hot Disk 2500-OT, Uppsala, Sweden). Based on this method, a disk-shaped sensor with the diameter of 2.005 mm was fixed between two round samples with the diameter of 25 mm and thickness of 4 mm. P2400 SiC paper was used to smooth the composite's surface for the purpose of good thermal contact. The dielectric properties were characterized by a broad frequency dielectric spectrometer Concept 50 (NOVOCONTROL, Germany). The diameter and thickness of the samples were respectively 25 mm and 4 mm.

2.5. Characterization

3. Results and discussion

Scanning electron microscopy (SEM) images of h-BN and modified h-BN were obtained by using the scanning electron microscopy (SEM, Inspect F, FEI Company, USA). The cryo-fractured surfaces of epoxy/h-BN, epoxy/h-BN@rGO and epoxy/h-BN/rGO nanocomposites were characterized to confirm the morphology. The fractured surfaces were prepared in liquid N2 and were sputtered with gold in vacuum prior to observation. The morphology status of h-BN@GO was also observed with transmission electron microscope (TEM, JEF-2100F, jeol) at an acceleration voltage 200 KV. Elemental mapping were utilized to analyze the composition of elements and their dispersion in h-BN and modified h-BN to

3.1. Electrostatic self-assembly of GO and modified h-BN

2.4. Preparation of epoxy-based composites

Graphene oxide, the derivative of graphene, can ionized negative charge in aqueous solution due to abundant carboxylic acid groups on its surface. It was reported that this ultrathin and twodimensional sheet could coassembly with positively charged particle in aqueous solution through mutual electrostatic interactions [34e36]. However, for commercially available h-BN, the flat surfaces are molecularly smooth and there is no functional groups on them except for the trace amount of hydroxyl groups and amino groups on the edge planes [37,38]. Zeta potential value of neat h-BN (15.04 mV) indicated impossible self-assembly with negatively

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charged GO (57.79 mV). Hence, positively charged functional group such as eNH2 needs to be grafted onto the surface of h-BN. Hence, firstly, inert h-BN needs to be hydroxylated to generate numerous eOH on its surface. Observed from the photographs in Fig. 1, after this intense oxidation described in the experimental section, the color of h-BN has obviously changed from white to brown yellow. The relevant results and detailed discussions about oxidation of h-BN will be published elsewhere in the near future, hence only the important XPS results were provided here in Fig. 2. One can observe that an obvious enhancement in the signal intensity of the O1S peak was exhibited and the content of O increased from almost 1.20 to 10.26%, suggesting abundant eOH groups have been generated not only at the edge, but also on the surface of h-BN. From Fig. 2 (b) and (c), the characteristic peak B1s (O) of h-BN (HNO3) marked with blue line increased significantly in comparison with that of raw h-BN, further indicating these large amount eOH were mostly linked to B atoms after oxidation. After successful hydroxylation to h-BN, secondly, the modified h-BN was grafted with APS, aiming at introducing eNH2. Zeta potential characterization in Fig. 3 shows that positively charged hBN-NH2 (x potential ¼ þ36.15 mV) was collected, meaning APS was successfully grafted and the possibility of self-assembly with GO in the aqueous solution. Thirdly, this h-BN-NH2 was utilized to coassembly with GO in aqueous solution in order to make GO nanosheets be absorbed on the surface of h-BN. According to the photographs in Fig. 1, it was clear that homogeneous clay brown powders have been collected, suggesting successful absorption of GO. In comparison with h-BN, the similar characteristic peaks of GO plotted in the Raman spectra (Figure S2) also confirmed above conclusion as well. To further verify the coassembly, SEM was also applied to observe the morphology of this hybrid filler and the results were shown in Fig. 4 (a)-(h). According to Fig. 4 (a), the GO nanosheets with the medium lateral dimension of ~5 mm was selected. And in order to control the dispersion of GO on the h-BN, larger h-BN with the lateral size of 30e40 mm was chosen. In this case, the dispersion and morphology of GO on the surface of h-BN can be controlled to prevent direct contact among GO sheets. Furthermore, one can observe that no matter for raw h-BN or h-BN modified by HNO3, their surfaces were both smooth, with a few pony-sized but relatively thick h-BN sheets overlaid on them (Fig. 4 (b), (c) and (d)). However, the images plotted in Fig. 4 (e) and (f) exhibited that ultra-thin and flexible sheets were tightly fixed on the surface of h-BN, suggesting the successful immobilization of GO for preparing h-BN@GO. However, GO is so flexible and ultra-thin that it is difficult to be clearly observed by SEM unless being reduced. After being exposed to chemical reduction by DDM at the same conditions in which we fabricated the epoxy nanocomposites, the flexible GO sheets have been converted into crisp thin lamella that were still firmly fixed on the surface, indicating not only the reducing capacity of DDM but also clear absorption of rGO. However, by means of SEM characterization, we still cannot observe the clear morphology and dispersion status of GO on h-BN well. Hence, TEM was further used. As was shown in Fig. 5, two different h-BN@GO specimens were randomly selected to be characterized. One can only see from Fig. 5 (a) and (b) that ultra-thin GO was absorbed at the edge of h-BN, but it seemed to be difficult to be observed at the surface of h-BN owing to the relatively thick layer which cannot be transmitted by electron beam. However, the second h-BN@GO plotted in Fig. 5 (c) and (d) was fortunately transmitted, and some GO folds can be scrutinized on the surface, suggesting most surface area of h-BN was occupied by GO. What's more, for the sake of making distribution of GO on h-BN certain, element mapping was characterized as well. Elemental mapping plotted in Fig. 5 (e) by means of EDX exhibited uniform distribution of B, N and C, confirming that GO mostly

dispersed not at the edge of h-BN but on the lateral surface. Moreover, the distribution of O showed local agglomeration, suggesting not continues but segregated GO sheets absorbed on h-BN. Finally, TGA (Figure S4) was applied to calculate that the absorbed GO content was about 1.6 wt%, and the content of GO was also confirmed by the results plotted in Figure S3. In conclusion, such a high content of h-BN in the hybrid filler with flexible and ultra-thin GO absorbed on the surface was successfully prepared, and it must

Fig. 2. XPS spectra of h-BN and h-BN modified by HNO3. (a) XPS wide spectra of h-BN and h-BN modified by HNO3; (b) XPS B1s core-level spectra of h-BN, B1s (N) is purple, B1s (O) is blue; (c) XPS B1s core-level spectra of h-BN modified by HNO3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 3. Zeta potential of (a) h-BN, (b) h-BN modified by HNO3, (c) h-BN-NH2 and (d) GO.

result in high thermal conductivity and good dielectric properties when they are introduced into polymer matrix and be completely reduced. 3.2. Dielectric properties Although it is generally accepted that amine chemical agents such as hydrazine hydrate [39], ethylenediamine [40], or ammonia borane [41] are efficient to reduce GO, the completely reduced rGO

on h-BN sheets tend to make them agglomerate in the epoxy matrix as due to poor interactions. Hence amine curing agent (DDM) was selected as the reductive agent to in-situ reduce the GO in the epoxy matrix when it was cured at the high temperature, for the purpose of realizing good dispersion and excellent interactions through chemical bonds between DDM and rGO. The results of XPS, FTIR, Raman spectra in the support information all demonstrate that GO has been successfully chemical reduced by curing agent DDM.

Fig. 4. Typical SEM images of (a) GO, (b) neat h-BN, (c, d) h-BN modified by HNO3, (e, f) h-BN@GO and (g, h) h-BN@rGO reduced by curing agent.

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Fig. 5. TEM images of randomly selected h-BN@GO with different magnifications. (a, c) low resolution; (b, d) high resolution; (e) Elemental mapping of h-BN@GO: B (red), N (green), C (yellow), O (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

After demonstrating the successful absorption of GO on h-BN and the effective reductive effect of DDM on GO, this hybrid filler prepared by self-assembly was added into epoxy and exposed to chemical reduction by DDM during curing process. Then, the dielectric properties of epoxy/h-BN@rGO was tested in comparison with those of epoxy/h-BN, and the results were shown in Fig. 6. It is found that the epoxy/h-BN composites exhibit weak frequency dependence. But for epoxy/h-BN@rGO, weak frequency dependence of dielectric constant and dielectric loss are only obtained at the high mass fraction (>20 wt%), especially at the frequency range from 102~105 Hz. Moreover, it is noted that a high dielectric constant of ~15 and an extremely low dielectric loss of 0.0068 were obtained for the nanocomposites containing 30 wt% h-BN@rGO (~18 vol%). The enhancement of dielectric constant is relatively higher than that of epoxy filled with the same volume concentration of BaTiO3 [42] and the value of dielectric loss is much lower even compared to that of neat epoxy. In order to study the variation of dielectric constant and dielectric loss of epoxy composites as function of the fillers loading, both the properties at the low frequency of 100 Hz were plotted in Fig. 7. One can observe that as increase of h-BN content, the dielectric constant was almost consistent with that of neat epoxy. But for h-BN@rGO, dielectric constant exhibited enormous enhancement with increasing hybrid filler content. Compared to dielectric constant, it is interesting that the dielectric loss of epoxy/h-BN was dramatically reduced with increasing h-BN loading, as was shown in Fig. 7 (b). It is generally considered that the addition of inorganic fillers, such as SiC [43], AlN [44], Al2O3 [45], into insulating matrix often leads to high dielectric loss and the dielectric loss of composites usually increases as increase of filler concentration. In this study, the situation was interesting and unusual, and similar result was observed for exfoliated h-BN in the previous report [8]. It is well-known that the dielectric loss is closely related to electrical conductivity of the composites. In terms of h-BN or exfoliated h-BN, they possesses extremely low electrical conductivity, hence embedded h-BN or exfoliated h-BN in the epoxy matrix could inhibit the mobility of charge carrier. And more h-BN was introduced, more volume fraction of interfaces between h-BN and matrix was generated, thus lower dielectric loss was obtained. Moreover, in this epoxy/hBN@rGO system, it is striking to be noted that through selfassembly and absorption of GO on h-BN, this nanocomposites

Fig. 6. Frequency-dependent (a) dielectric constant and (b) Tan Deta of epoxy composites as function of the fillers loading.

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exhibited similar decreased tendency as increase of the hybrid filler content. This phenomenon has been never seen in the conductive filler filled system, and the mechanism will be discussed as follows. 3.3. Mechanism for dielectric properties

Fig. 7. Variation of (a) dielectric constant and (b) Tan Deta of epoxy composites as function of the fillers loading at 100 Hz.

Before proposing the mechanism to explain the improved dielectric properties, SEM was characterized to observe the morphology and structure of the nanocomposites because the structure usually plays an important role in affecting relevant properties (Fig. 8) [46,47]. Since intense mechanical stirring was applied to disperse fillers, uniform distribution of h-BN in epoxy matrix for all the three different systems was achieved according to Fig. 8 (a), (c) and (e). However, in terms of rGO, the dispersed situations were extremely different. Compared to neat h-BN whose surface was smooth, the surfaces of h-BN@rGO usually exhibited rough appearance with several small-sized ultra-thin rGO sheets coated on them (Fig. 8 (d)). However, crisp rGO sheets seemed to be not firmly absorbed but peeled off from the surface of h-BN. It is understandable that when GO was reduced by DDM, the covalent bonds were formed between matrix and rGO, resulting in stronger interaction than electrostatic interaction between rGO and modified h-BN. Hence, once the composites were broken at the fracture interface in liquid nitrogen, the original rGO sheets which was firmly fixed on the surface between epoxy and h-BN tended to be exfoliated from h-BN, inevitably leading to above observation. On the other hand, for epoxy/h-BN/rGO, rGO sheets were inclined to be excluded from h-BN dominant areas, so the appearance of matrix seemed to be bumpy, with numerous rigid rGO sheets embedded in it (Fig. 8 (e) and (f)). But for epoxy/h-BN or epoxy/h-BN@rGO, no rGO sheets dispersed in vacant epoxy matrix regions, leading to smooth observations from SEM images. Concluded from above SEM observations and dielectric properties, a mechanism based on a micro-capacitor network model [13,48,49] was proposed. According to the model, rGO embedded in epoxy can be viewed as electrodes and epoxy matrix or h-BN between rGO sheets can be considered as dielectric, leading to abundant microcapacitors in the nanocomposites. And as increase of conductive rGO, the number of microcapacitors increases and the

Fig. 8. SEM images of (a, b) epoxy/30 wt% h-BN, (c, d) epoxy/30 wt% h-BN@rGO and (e, f) epoxy/30 wt% h-BN/rGO (The content of rGO is the same as that of epoxy/30 wt% hBN@rGO.).

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Fig. 9. Schematic representation of morphology and distribution of h-BN@rGO (left) and directly mixed h-BN/rGO (right) in epoxy matrix.

thickness of dielectric layers decreases, thus higher capacitance is obtained, resulting in higher dielectric constant. In epoxy/hBN@rGO systems, we can see as illustrated in Fig. 9 that these planar rGO sheets were all absorbed on the surface of flat h-BN. Hence the distribution of rGO in the epoxy matrix was limited by hBN substrate. In this case, not only was rGO agglomeration suppressed, but also direct contact of rGO was inhibited even though hBN sheets linked together at the high loading as due to the special two-dimensional structure of h-BN. Hence some different effects compared to that of randomly distributed rGO and h-BN in epoxy would be exerted on the dielectric properties, which were discussed as follows. For dielectric loss, it is well-known that three factors including electrical conductivity, interfacial polarization and relaxation play important role in affecting the dielectric loss of composites [50]. Among these, the electrical conductivity is the major factor as due to the same composition of the composites but distinct dispersion of rGO for epoxy/h-BN@rGO and epoxy/h-BNrGO. For epoxy/h-BN@rGO, on the one hand, the insulating h-BN network of 30 wt % could efficiently inhibit the mobility of charge carrier in epoxy, resulting in reduced electrical conductivity and consequent reduced dielectric loss (Figure S8). On the other hand, the separated rGO sheets which were fixed on the surface of h-BN cannot form conducting pathways, resulting in suppressed electrical conductivity compared to that of epoxy/h-BN-rGO, hence avoiding generating leakage current. As a result, ultra-low dielectric loss was obtained for epoxy nanocomposites filled with 30 wt% h-BN@rGO. However, for randomly dispersed h-BN and rGO in epoxy, the large amount of h-BN excluded rGO sheets from their dominant regions, leading to more possibilities for their direct contact. So relatively higher electrical conductivity and higher dielectric loss was obtained for epoxy/30 wt% h-BN-rGO (Figure S7 and Figure S8). The low dielectric loss is extremely significant for practical applications. Dielectric materials with higher dielectric loss will generate more heat at electric field, leading to ruin the performance and service life of the devices. Similarly, the higher dielectric constant of epoxy/h-BN@rGO than that of epoxy/h-BN/ rGO (Figure S7) can also be ascribed to two reasons discussed as follows. On the one hand, for h-BN@rGO, rGO agglomeration was mostly inhibited, bringing about more microcapacitors than that of randomly dispersed rGO in nanocomposites. On the other hand, the absorbed rGO sheets on the two sides of h-BN were almost parallel to each other, generating a special microcapacitor for a single

h-BN@rGO system. Hence the larger parallel areas and the smaller parallel distance, the higher capacitance for these microcapacitors. As consequences, relatively high dielectric constant was achieved for epoxy/h-BN@30 wt% rGO. Compared to dielectric materials reported in the previous, the dielectric constant of 15 is not very high. This was mostly limited by the absorbed content of rGO on h-BN. It is considered that the lateral size of GO, the oxidation degree and the thickness of h-BN play the important role in determining the concentration of rGO in h-BN@rGO hybrid filler. Hence, in the future work, these factors need to be further studied to enhance the dielectric constant while maintaining the low dielectric loss. 3.4. Thermal conductivity The capability of heat dissipation depending on thermal conductivity is usually an overlooked aspect for dielectric materials. However, as dielectric loss generates vast joule heat, effective dissipation of heat from dielectric parts to the external environment is critical for performances and operation life of electronic devices [51]. For this reason, the thermal conductivity of dielectric materials is worthwhile to be studied. It is find that, for most polymers such as epoxy in this study, the thermal conductivity is

Fig. 10. Thermal conductivity and thermal enhancement of epoxy/h-BN@rGO as function of filler content.

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approximately ~0.2 W/m K (Fig. 10), which makes polymer materials to be seriously poor thermal conductors. Introducing thermally conductive fillers such as h-BN, expanded graphite into the matrix is usually an efficient way [52]. With incorporation of hBN@rGO to neat epoxy, the thermal conductivity increased as increase of its concentration. When 30 wt% (18 vol%) h-BN@rGO were introduced, the thermal enhancement factor was 390% compared to neat epoxy, obtaining thermal conductivity of 0.94 W/m K which was much higher than polymers filled with same volume content of common dielectric fillers such as barium titanate [7]. Hence, this hybrid filler is believed to be effective to not only improve the dielectric properties of polymer matrix, but also largely enhancing the capability of heat dissipation. 4. Conclusion The h-BN@GO filler has been successfully prepared via electrostatic self-assembly, with ultra-thin GO sheets firmly absorbed on the lateral surface of h-BN. After mixed in epoxy matrix and exposed to chemical reduction, h-BN@rGO exhibited good abilities including not only largely enhancing the dielectric constant and thermal conductivity, but also reducing the dielectric loss. As consequences, the dielectric constant of ~15, the dielectric loss of 0.0068 and the thermal conductivity of 0.94 W/m K was obtained for epoxy filled with 30 wt% hybrid filler. And this special hybrid filler is believed to be applied in fabricating multi-functional dielectric materials with high dielectric constant, low dielectric loss as well as good capability of heat dissipation. Acknowledgements This work was supported by the National Natural Science Foundation of China (51421061 and 51210005). We would like to express our great thanks to Guangdong Shengyi Technology Limited Corporation (15H0613) for financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.compscitech.2016.08.015. References [1] H. Liu, Y. Shen, Y. Song, C.W. Nan, Y. Lin, X. Yang, Carbon nanotube array/ polymer core/shell structured composites with high dielectric permittivity, low dielectric loss, and large energy density, Adv. Mater. 23 (2011) 5104e5108. [2] Q. Zhang, H. Li, M. Poh, F. Xia, Z.-Y. Cheng, H. Xu, et al., An all-organic composite actuator material with a high dielectric constant, Nature 419 (2002) 284e287. [3] S.V. Ahir, E.M. Terentjev, Photomechanical actuation in polymerenanotube composites, Nat. Mater. 4 (2005) 491e495. [4] D. Lu, C. Wong, Materials for Advanced Packaging, Springer, 2009. [5] Q. Li, G. Zhang, F. Liu, K. Han, M.R. Gadinski, C. Xiong, et al., Solution-processed ferroelectric terpolymer nanocomposites with high breakdown strength and energy density utilizing boron nitride nanosheets, Energy Environ. Sci. 8 (2015) 922e931. [6] Q. Li, L. Chen, M.R. Gadinski, S. Zhang, G. Zhang, H. Li, et al., Flexible hightemperature dielectric materials from polymer nanocomposites, Nature 523 (2015) 576e579. [7] L. Xie, X. Huang, K. Yang, S. Li, P. Jiang, “Grafting to” route to PVDF-HFP-GMA/ BaTiO 3 nanocomposites with high dielectric constant and high thermal conductivity for energy storage and thermal management applications, J. Mater. Chem. A 2 (2014) 5244e5251. [8] X. Huang, C. Zhi, P. Jiang, D. Golberg, Y. Bando, T. Tanaka, Polyhedral oligosilsesquioxane-modified boron nitride nanotube based epoxy nanocomposites: an ideal dielectric material with high thermal conductivity, Adv. Funct. Mater. 23 (2013) 1824e1831. [9] K. Yang, X. Huang, Y. Huang, L. Xie, P. Jiang, Fluoro-polymer@ BaTiO3 hybrid nanoparticles prepared via RAFT polymerization: toward ferroelectric polymer nanocomposites with high dielectric constant and low dielectric loss for

199

energy storage application, Chem. Mater. 25 (2013) 2327e2338. [10] B. Luo, X. Wang, Y. Wang, L. Li, Fabrication, characterization, properties and theoretical analysis of ceramic/PVDF composite flexible films with high dielectric constant and low dielectric loss, J. Mater. Chem. A 2 (2014) 510e519. [11] J. Chang, G. Liang, A. Gu, S. Cai, L. Yuan, The production of carbon nanotube/ epoxy composites with a very high dielectric constant and low dielectric loss by microwave curing, Carbon 50 (2012) 689e698. [12] J.Y. Kim, W.H. Lee, J.W. Suk, J.R. Potts, H. Chou, I.N. Kholmanov, et al., Chlorination of reduced graphene oxide enhances the dielectric constant of reduced graphene oxide/polymer composites, Adv. Mater. 25 (2013) 2308e2313. [13] M. Li, X. Huang, C. Wu, H. Xu, P. Jiang, T. Tanaka, Fabrication of twodimensional hybrid sheets by decorating insulating PANI on reduced graphene oxide for polymer nanocomposites with low dielectric loss and high dielectric constant, J. Mater. Chem. 22 (2012) 23477e23484. [14] C. Wu, X. Huang, X. Wu, L. Xie, K. Yang, P. Jiang, Graphene oxide-encapsulated carbon nanotube hybrids for high dielectric performance nanocomposites with enhanced energy storage density, Nanoscale 5 (2013) 3847e3855. [15] S. Luo, S. Yu, R. Sun, C.P. Wong, Nano Ag-deposited BaTiO3 hybrid particles as fillers for polymeric dielectric composites: toward high dielectric constant and suppressed loss, ACS Appl. Mater. Interfaces 6 (2013) 176e182. [16] S. Stankovich, D.A. Dikin, G.H. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, et al., Graphene-based composite materials, Nature 442 (2006) 282e286. [17] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385e388. [18] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, et al., Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (2008) 902e907. [19] A.C. Ferrari, F. Bonaccorso, V. Fal'Ko, K.S. Novoselov, S. Roche, P. Bøggild, et al., Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, Nanoscale 7 (2015), 4598e4810. [20] P. Fan, L. Wang, J. Yang, F. Chen, M. Zhong, Graphene/poly (vinylidene fluoride) composites with high dielectric constant and low percolation threshold, Nanotechnology 23 (2012) 365702. [21] N. Yousefi, X. Sun, X. Lin, X. Shen, J. Jia, B. Zhang, et al., Highly aligned graphene/polymer nanocomposites with excellent dielectric properties for highperformance electromagnetic interference shielding, Adv. Mater. 26 (2014) 5480e5487. [22] K. Yang, X. Huang, L. Fang, J. He, P. Jiang, Fluoro-polymer functionalized graphene for flexible ferroelectric polymer-based high-k nanocomposites with suppressed dielectric loss and low percolation threshold, Nanoscale 6 (2014) 14740e14753. [23] C. Wu, X. Huang, L. Xie, X. Wu, J. Yu, P. Jiang, Morphology-controllable graphene-TiO 2 nanorod hybrid nanostructures for polymer composites with high dielectric performance, J. Mater. Chem. 21 (2011) 17729e17736. [24] T. Wang, G. Liang, L. Yuan, A. Gu, Unique hybridized graphene and its high dielectric constant composites with enhanced frequency stability, low dielectric loss and percolation threshold, Carbon 77 (2014) 920e932. [25] Y. Gao, A. Gu, Y. Jiao, Y. Yang, G. Liang, J. t Hu, et al., High-performance hexagonal boron nitride/bismaleimide composites with high thermal conductivity, low coefficient of thermal expansion, and low dielectric loss, Polym. Adv. Technol. 23 (2012) 919e928. [26] Z. Liu, Y. Gong, W. Zhou, L. Ma, J. Yu, J.C. Idrobo, et al., Ultrathin hightemperature oxidation-resistant coatings of hexagonal boron nitride, Nat. Commun. (2013) 4. [27] Z. Lin, A. Mcnamara, Y. Liu, K.-s. Moon, C.-P. Wong, Exfoliated hexagonal boron nitride-based polymer nanocomposite with enhanced thermal conductivity for electronic encapsulation, Compos. Sci. Technol. 90 (2014) 123e128. [28] M. Donnay, S. Tzavalas, E. Logakis, Boron nitride filled epoxy with improved thermal conductivity and dielectric breakdown strength, Compos. Sci. Technol. 110 (2015) 152e158. [29] X. Cui, P. Ding, N. Zhuang, L. Shi, N. Song, S. Tang, Thermal conductive and mechanical properties of polymeric composites based on solution-exfoliated boron nitride and graphene nanosheets: a morphology-promoted synergistic effect, ACS Appl. Mater. Interfaces 7 (2015) 19068e19075. [30] A. Pakdel, Y. Bando, D. Golberg, Plasma-assisted interface engineering of boron nitride nanostructure films, ACS Nano 8 (2014) 10631e10639. [31] T. Sainsbury, A. Satti, P. May, Z. Wang, I. McGovern, Y.K. Gun’ko, et al., Oxygen radical functionalization of boron nitride nanosheets, J. Am. Chem. Soc. 134 (2012) 18758e18771. [32] C. Zhi, Y. Bando, T. Terao, C. Tang, H. Kuwahara, D. Golberg, Chemically activated boron nitride nanotubes, Chem. Asian J. 4 (2009) 1536e1540. [33] W.S. Hummers Jr., R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958), 1339e1339. [34] S. Yang, X. Feng, S. Ivanovici, K. Müllen, Fabrication of graphene-encapsulated oxide nanoparticles: towards high-performance anode materials for lithium storage, Angew. Chem. Int. Ed. 49 (2010) 8408e8411. [35] L. Chen, S. Chai, K. Liu, N. Ning, J. Gao, Q. Liu, et al., Quaternary nanocomposites consisting of graphene, Fe3O4@ Fe core@ shell, and ZnO nanoparticles: synthesis and excellent electromagnetic absorption properties, ACS Appl. Mater. Interfaces 4 (2012) 4398e4404. [36] X. Zhou, Y.X. Yin, L.J. Wan, Y.G. Guo, Self-assembled nanocomposite of silicon nanoparticles encapsulated in graphene through electrostatic attraction for

200

K. Wu et al. / Composites Science and Technology 134 (2016) 191e200

lithium-ion batteries, Adv. Energy Mater. 2 (2012) 1086e1090. [37] M.T. Huang, H. Ishida, Investigation of the boron nitride/polybenzoxazine interphase, J. Polym. Sci. B Polym. Phys. 37 (1999) 2360e2372. [38] K. Sato, H. Horibe, T. Shirai, Y. Hotta, H. Nakano, H. Nagai, et al., Thermally conductive composite films of hexagonal boron nitride and polyimide with affinity-enhanced interfaces, J. Mater. Chem. 20 (2010) 2749e2752. [39] P.-G. Ren, D.-X. Yan, X. Ji, T. Chen, Z.-M. Li, Temperature dependence of graphene oxide reduced by hydrazine hydrate, Nanotechnology 22 (2010) 055705. [40] J. Wang, S. Liang, L. Ma, S. Ding, X. Yu, L. Zhou, et al., One-pot synthesis of CdSreduced graphene oxide 3D composites with enhanced photocatalytic properties, Crystengcomm 16 (2014) 399e405. Kim, J. Suka Chung, Highly [41] V. Hung aPham, S. Hyun aHur, E. Jung aKim, B. Sunga efficient reduction of graphene oxide using ammonia borane, Chem. Commun. 49 (2013) 6665e6667. [42] S.-D. Cho, S.-Y. Lee, J.-G. Hyun, K.-W. Paik, Comparison of theoretical predictions and experimental values of the dielectric constant of epoxy/BaTiO3 composite embedded capacitor films, J. Mater. Sci. Mater. Electron. 16 (2005) 77e84. [43] W. Zhou, D. Yu, C. Min, Y. Fu, X. Guo, Thermal, dielectric, and mechanical properties of SiC particles filled linear low-density polyethylene composites, J. Appl. Polym. Sci. 112 (2009) 1695e1703. [44] S. Yu, P. Hing, X. Hu, Dielectric properties of polystyrene-aluminum-nitride composites, J. Appl. Phys. 88 (2000) 398e404. [45] Y.-P. Zheng, J.-X. Zhang, Q. Li, W. Chen, X. Zhang, The influence of high content nano-Al2O3 on the properties of epoxy resin composites, Polym. Plast. Technol. Eng. 48 (2009) 384e388. [46] Y. He, Y. Guo, R. He, T. Jin, F. Chen, Q. Fu, Towards high molecular weight poly (bisphenol a carbonate) with excellent thermal stability and mechanical properties by solid-state polymerization, Chin. J. Polym. Sci. 33 (2015)

1176e1185. [47] B. Su, Y. Zhao, F. Chen, Q. Fu, Effect of microdomain structure on the mechanical behavior of binary blends, Chin. J. Polym. Sci. 33 (2015) 964e975. [48] X. Huang, P. Jiang, L. Xie, Ferroelectric polymer/silver nanocomposites with high dielectric constant and high thermal conductivity, Appl. Phys. Lett. 95 (2009) 242901. [49] Z.M. Dang, L. Wang, Y. Yin, Q. Zhang, Q.Q. Lei, Giant dielectric permittivities in functionalized carbon-nanotube/electroactive-polymer nanocomposites, Adv. Mater. 19 (2007) 852e857. [50] Y. Li, X. Huang, Z. Hu, P. Jiang, S. Li, T. Tanaka, Large dielectric constant and high thermal conductivity in poly (vinylidene fluoride)/barium titanate/silicon carbide three-phase nanocomposites, ACS Appl. Mater. Interfaces 3 (11) (2011) 4396e4403. [51] L. Fang, W. Wu, X. Huang, J. He, P. Jiang, Hydrangea-like zinc oxide superstructures for ferroelectric polymer composites with high thermal conductivity and high dielectric constant, Compos. Sci. Technol. 107 (2015) 67e74. [52] K. Wu, Y. Xue, W. Yang, S. Chai, F. Chen, Q. Fu, Largely enhanced thermal and electrical conductivity via constructing double percolated filler network in polypropylene/expanded graphiteeMulti-wall carbon nanotubes ternary composites, Compos. Sci. Technol. 130 (2016) 28e35. [53] K.K. Sadasivuni, D. Ponnamma, B. Kumar, et al., Dielectric properties of modified graphene oxide filled polyurethane nanocomposites and its correlation with rheology, Compos. Sci. Technol. 104 (2014) 18e25. [54] H. Li, Z. Chen, L. Liu, et al., Poly (vinyl pyrrolidone)-coated graphene/poly (vinylidene fluoride) composite films with high dielectric permittivity and low loss, Compos. Sci. Technol. 121 (2015) 49e55. [55] H.B. Cho, T. Nakayama, H. Suematsu, et al., Insulating polymer nanocomposites with high-thermal-conduction routes via linear densely packed boron nitride nanosheets, Compos. Sci. Technol. 129 (2016) 205e213.